Ultraviolet semiconductor light-emitting elements that emit light within a UV wavelength range are expected to find application to a variety of fields including sanitary and hygiene, medicine, industry, illumination, precision machinery, and the like.
However, typical ultraviolet semiconductor light-emitting elements using a nitride semiconductor material for a light-emitting layer presently have an emission efficiency and a light output lower than those of nitride semiconductor light-emitting elements that emit blue light. Therefore, the typical ultraviolet semiconductor light-emitting elements have not yet found wide industrial application.
A low emission efficiency of ultraviolet semiconductor light-emitting elements can be attributed to a high density of threading dislocations, predominant nature of non-emitting recombination, and a low internal quantum efficiency or to insufficient performance of a p-type nitride semiconductor layer. In particular, the major reason is a low take-out efficiency of the emitted ultraviolet light to the outside. For example, there has been an ultraviolet semiconductor light-emitting element in which AlxGa1−xN (x>=0.4) is used as a material for the light-emitting layer, and then a p-type GaN layer that can ensure a comparatively high hole concentration is provided as a p-type contact layer for obtaining ohmic contact with a p-electrode in the p-type nitride semiconductor layer (see Non-Patent Document 1). In the ultraviolet semiconductor light-emitting element, since the p-type GaN layer absorbs ultraviolet light with a wavelength equal to or less than 360 nm, ultraviolet light incident upon the p-type GaN layer is absorbed and cannot be taken out, thereby reducing the efficiency of light take-out.
By contrast, an ultraviolet semiconductor light-emitting element has been suggested which has a laminated structure including an n-type nitride semiconductor layer, a light-emitting layer constituted by AlxGa1−xN (0.4=<x=<1.0), and a p-type nitride semiconductor layer on one surface side of a substrate. The p-type nitride semiconductor layer includes a p-type clad layer constituted by an Aly2Ga1−y2N (x<y2=<1.0) layer that is richer in Al than the light-emitting layer, and a p-type contact layer constituted by an Alz2Ga1−z2N (0=<z2<y2) layer located on the p-type clad layer. In the ultraviolet semiconductor light-emitting element, a groove section is formed in the p-type contact layer, and ultraviolet light can be taken out from the groove section of the p-type contact layer (see Patent Document 1).